1. Field of the Invention
The present invention relates to a video camera, and more particularly to a video camera with automatic white balance control function.
2. Related Background Art
FIG. 1 is a block diagram showing principal parts of a video camera of the above-mentioned kind. Light 1 from an object enters, through a lens 2, the light-receiving part of an image pickup device 3, and is photoelectrically converted into electrical color signals of R' (red), G' (green) and B' (blue). Among the color signals from said image pickup device 3, the R' and B' signals are respectively amplified by variable-gain amplifiers 4a, 4c of a voltage control system for white balance control, while the G' signal is amplified by an ordinary amplifier 4b, and R, G and B color signals obtained by such color balance correction are supplied to a signal processing circuit 5. Said circuit 5 effects necessary signal processing, and sends a luminance signal Y and color difference signals R-Y, B-Y to an encoder circuit 6, which in response generates a composite color video signal.
On the other hand, said color difference signals R-Y, B-Y from the signal processing circuit 5 are also supplied, respectively through analog-to-digital (A/D) converters 7a, 7b, to a microprocessor 8 which effects white balance control based on thus entered color difference signals R-Y, B-Y. More specifically, white balance control signals are supplied, through digital-to-analog (D/A) converters 9a, 9b, to the variable-gain amplifiers 4a, 4c in such a manner that said R-Y and B-Y signals become "0". The white balance control is achieved by the control of the gains of said variable-gain amplifiers 4a, 4c.
Switch circuit 10 is connected to the microprocessor 8, and, when a switch 10a of said switch circuit 10 is turned on, the microprocessor 8 is activated to close the circuit of the white balance control system, thereby effecting white balance control.
FIG. 2 is a flow chart of a conventional white balance control sequence in the video camera of the above-explained structure.
When the white balance control operation is started by the closing of the switch 10a, a step S1 effects the initialization of the microprocessor 8 shown in FIG. 1, and a step S2 sets a timer in the microprocessor 8. Said timer is used for preventing prolonged continuation of the white balance control operation in case the white balance is not attained for some reason, but is set at a relatively long time. Then a step S3 discriminates whether the above-mentioned color difference signals R-Y, B-Y are "0". If said signals are "0", a white balanced state is identified to have been already reached and the white balance control operation is terminated. On the other hand, if said signals are not "0", a step S4 calculates an R control amount and a B control amount respectively for the variable-gain amplifiers 4a, 4c for the R and B signals, and a step S5 discriminates whether these control amounts (target control values) are within a predetermined white balance control range, namely within a control table stored in the microprocessor 8, and, if said control amounts are contained in the control table, a step S7 supplies the variable-gain amplifiers 4a, 4c with R and B control signals corresponding to thus calculated control amounts. However, if the step S5 identifies that the target control values are not contained in the control table, a step S6 substitutes the R and B control values with limit values of the control table closest to said target control values, and the step S7 releases R and B control signals corresponding to thus substituted control value. Then a step S8 discriminates whether the set time of said timer has expired, and, if not, the sequence returns to the step S3 to repeat the sequence thereafter, but, if expired, the R and B control signals at this point are released and the white balance control operation is terminated.
FIG. 3 shows an example of the white balance control range mentioned above, wherein an area surrounded by a solid line (i) corresponds to the above-mentioned table. A broken line (ii) in said table (i) indicates the trajectory of white balance attained with a black body radiation light source, and the position of the white balance varies according to the color temperature of said light source. The table (i) is wider than said trajectory (ii) corresponding to the light of black body radiation, in order to attain the white balance also for artificial light sources such as a fluorescent lamp. However the width of said table (i) is limited to a practically acceptable value since, if it is too wide, the color vanishes when the white balance is obtained on a monotonously colored object.
However, in such conventional white balance control device, if the white balance control is conducted under a light source outside the color temperature range corresponding to the black body radiation or a special artificial light source, or on an object with a large area of single color, the color difference signals cannot be reduced to zero at the same time because the control signals cannot be varied beyond the limit value of said table. Consequently the white balance control operation is continued until the set time of said timer expires, and requires a longer time than in the white balance determining operation.
More specifically, if the control position at the start of white balance control operation is at a point A in the table (i) shown in FIG. 4 and the target control position is at B outside the table (i), the control operation stops when the control value reaches a boundary point C of the table (i) closest to the target control position, and the white balance control operation is thereafter continued until the set time of the timer expires, though the control value no longer varies.
Also as the above-explained conventional structure is based on the white balance control of so-called TTL system employing the video signal obtained from the object through the image pickup device as the control data for the white balance control operation, such operation becomes impossible in principle for a special object, for example an entirely red object. This is because the average of the image signal obtained from such object in a field period is not gray.
As a specific example, if the automatic white balance control operation is conducted while a red paper is caught over the entire image frame, the signal processing circuit 6 tries to reproduce the red object as a gray image, whereby the color of the image displayed on the monitor becomes drastically different from the read color actually observed by the user.
In order to overcome such drawback there can be utilized for example an external light metering sensor system, but the use of such external sensor system complicates the structure and the handling and elevates the cost of the apparatus.